Lateralization of auditory function is a salient, biologically-important phenomenon. Auditory areas of left temporal lobe are specialized for temporal processing of rapidly changing, broadband sounds and the right auditory areas for spectral analysis of tonal and steady-state signals. Distinct brain areas for these processes allow the human auditory system to perform accurate spectral analysis and precise temporal analysis instantly and simultaneously with sacrificing one to the other. Auditory function may also differ in a similar manner based on the ear stimulated. Objectives of this study are to 1) further define the acoustic characteristics auditory stimuli that dictate laterality 2) characterize the relationship between laterality of auditory function the laterality of activation of the auditory cortex and 3) determine how cortical laterality is reflected in subjects with unilateral sensory hearing loss and how that function varies by affected ear. Subjects will be normally hearing, right handed, young adults and those with unilateral hearing loss (UHL). An equal number of males and females will be evaluated. Measurements will be made for right, left and binaural stimulation (normals) or hearing ear (UHL). Five auditory experiments will be performed: frequency discrimination and intensity discrimination for a range of frequencies and tone durations, gap detection thresholds, temporal modulation transfer functions and speech (sentence) thresholds in noise. Each task will be performed using (A) standard psychophysical measures and (B) event related potentials in an oddball paradigm with high density (64 channel) EEG recordings. Analysis for (A) will include performance by ear. Performance on single ears of unilateral subjects will be compared in left and right deaf and to the normally hearing subjects. Electrophysiologic recordings (B) will be compared across hemispheres at homologous electrode sites for latency and amplitude as well as surface topography. Dipole source analysis will be applied to determine symmetry of activation. This approach will allow for comparison of laterality at a variety of levels of the auditory system, by subject response pattern and by physiologic response. We hypothesize that, 1) for normally hearing subjects, unilateral stimulation may accentuate the processing capacity seen in the opposite hemisphere 2) subjects with unilateral hearing loss will have symmetric cortical activation and be significantly disadvantaged in functions specialized to the hemisphere contralateral to the deafness. This information can be applied to improve current strategies for remediation of persons with hearing loss, such as cochlear implants and hearing aids by applying principles of naturally occurring laterality. PUBLIC HEALTH RELEVANCE A clear understanding of how the left and right ears work in individual fashion to allow for simultaneous frequency (left ear) and timing (right ear) analysis of sound could have a dramatic impact on the approach to providing aid to those with hearing impairment. Devices that process sound to be delivered to individual ears, such as cochlear implants or hearing aids, could be fit in ways that take advantage of the natural processing differences of the two sides of the brain. This project will help to reveal the true nature of the disability imparted to individuals with unilateral deafness and how such disability relates to the side of the deafness.